Question

At its melting point ice is lighter than water because

• A. ${H_2O}$ molecules are more closely packed in solid state
• B. Ice crystals have hollow hexagonal arrangement of ${H_2O}$ molecules.
• C. On melting of ice the ${H_2O}$ molecule shrinks in size
• D. Ice froms mostly heavy water on first melting.

Answer 1

The Correct Option is B

The question asks why ice is lighter than water at its melting point. To understand this, we must examine the structural differences between ice (solid water) and liquid water:

1. Structure of Ice: In the solid state, ice crystals form a lattice structure where the ${H_2O}$ molecules are arranged in a hexagonal pattern. This arrangement leads to a lattice that is less dense than the liquid phase. The hexagonal shape creates a structure that includes a significant amount of open space between molecules.
2. Density Implication: Because there are open spaces within the ice lattice, it occupies more volume than when the same molecules are in the liquid state. This leads to a decrease in density. Density is defined as mass per unit volume, and since the volume increases while the mass remains the same, the structure results in ice being less dense than water.
3. Reason for Lightness: Due to this less dense configuration, ice floats on water, meaning it is lighter than water on a per volume basis at the melting point.

Thus, the correct answer to the question provided is: Ice crystals have hollow hexagonal arrangement of ${H_2O}$ molecules.

Let's briefly analyze the other options to understand why they are incorrect:

• ${H_2O}$ molecules being more closely packed in the solid state is incorrect, as ice has a structure that entails more space between molecules than in liquid water, leading to lower density.
• On melting, the ${H_2O}$ molecules do not shrink but rather rearrange into a more densely packed structure. Hence, this option is incorrect.
• The idea that ice forms mostly heavy water on first melting is scientifically inaccurate within the context of pure ice and water, making this option incorrect.

In conclusion, the unique hexagonal lattice structure of ice is responsible for its ability to float on water, which explains why the answer is due to the hollow hexagonal arrangement of ${H_2O}$ molecules.